Field of the Invention
This invention relates to a substrate unit for a liquid discharging head which discharges liquid, method for producing the same, liquid discharging head incorporating the substrate unit, cartridge which monolithically combines the liquid discharging head and liquid tank holding a liquid to be sent to the liquid discharging head, and image forming apparatus which forms an image on a printing medium. They are applicable to general printing apparatuses, copiers, facsimiles having a communication system, word processors or the like having a printing section, industrial recording apparatuses combined with one or more varying processing units, and various devices, such as those for textile printing and etching.
xe2x80x9cPrintingxe2x80x9d or xe2x80x9crecordingxe2x80x9d used in this specification includes forming information of meaning, e.g., letters or patterns, and also forming a variety of images, patterns or the like on a printing medium, whether or not they have a meaning or recognizable by visual sense of a human. It also includes etching and other processing methods.
The xe2x80x9cprinting mediumxe2x80x9d used in this specification is not limited to paper or the like to be printed by general printers, but includes fabrics, plastic films, metallic plates, glass sheets, ceramics, lumbers, leathers or the like which can receive an ink. Its shape is not limited, but includes three-dimensional objects, e.g., spheres and cylinders, in addition to sheet-shaped objects.
The xe2x80x9cliquidxe2x80x9d used in this specification should be interpreted broadly, as is the case of definition of the above-described xe2x80x9cprinting or recording,xe2x80x9d and includes those for forming images, patterns or the like on a printing medium, etching of a printing medium, and processing of ink, e.g., solidification or insolubilization of a colorant in a ink attached to a printing medium.
Of the various printing methods known so far, liquid jet printing method (hereinafter referred to as ink jet printing method) is a very useful method for various reasons, e.g., nonimpact type printing which produces little noise while being in service, highspeed printing, and capacity of printing a common paper without needing a special fixation treatment.
This ink jet printing method flies droplets of an ink or a treatment solution for adjusting printability of an ink on a printing medium (hereinafter referred to generically as ink) by a varying working principle onto a printing medium, e.g., paper, for printing. The basic principle, as described by Japanese Patent Application Laid-Open No. 54-59936, is outlined below. In the ink jet printing method, thermal pulses are given as the information signal to an ink in an ink chamber capable of holding an ink, thereby discharging and flying the ink in the form of droplets by the force generated as a result of vaporization/expansion of the ink through a discharge port connected to the ink chamber onto a printing medium for printing.
This method has various advantages. It is suitable for high-speed prints and color prints, when a high-density, multi-array structure is used. The printer structure therefor can be simpler than the conventional one, making the printing head, i.e., ink jet head, compacter as a whole. Such heads are suitable for mass production, and may be elongated by fully utilizing IC and microwave processing techniques, which have been greatly advanced in level and reliability for the semiconductor industry. As such, it is applicable to wide areas.
A characteristic ink jet head of the ink jet printer for the ink jet printing method is provided with thermal energy generating means for forming flying droplets of ink discharged from the discharge port. It is considered that the thermal energy generating means is preferably designed to come into direct contact with the ink, for efficiently acting the energy on the ink and enhancing response of the ink jet head to the ON-OFF thermal actions.
The thermal energy generating means for an ink jet head is basically comprises a heat generating resist (electrothermal transducer) layer and a pair of electrode circuits for supplying electricity to the layer. Such a design may cause various problems, when the resist layer directly comes into contact with the ink, e.g., the ink may pass electricity, depending on its electrical resistance, to possibly cause electrolysis of the ink itself, or the energized resist layer may react with the ink on supplying electricity to the heat generating resist layer, to possibly cause corrosion of the resist layer to change its resist and eventual failure or breakdowns.
Therefore, various methods have been proposed to solve the above problems and thereby to improve reliability and durability of the resist layer for repeated used. For example, the resist layer is made of an inorganic material of relatively good characteristics for heat generating resist layer, e.g., alloy such as Ni or Cr, or metal boride such as ZrB2 or HfB2. The resist layer may be coated with a protective layer of an oxidation-resistant compound, e.g., SiO2, to positively prevent it from directly coming into contact with the ink.
It is a normal practice, when thermal energy generating means for an ink jet head is produced, to coat a heat generating resist layer, formed on a given substrate, with electrode circuits and a protective layer in this order. The protective layer is required to uniformly cover the resist layer to fully satisfy the requirements, i.e., prevention of failure of the resist layer and short-circuit between the resist layer and electrode wiring. In addition, it should be free of defects, e.g., pinholes.
Normally, the protective layer is further coated with a second, relatively thin protective layer, in order to securely cut off the protective layer from the ink. The thin second layer is normally of a metal, e.g., Ta, formed by sputtering. This second protective layer prevents inflow of ink, even when the first protective layer of SiO2, SiN or the like is cracked by repeated exposure to heat, generated in the heat generating resist layer. It also protects the resist layer from cavitation, resulting from foaming and defoaming cycles, to improve durability of the layer for repeated use.
However, the second protective layer may cause cracking of the first protective layer below, because of stresses therein being different from each other. Therefore, the second protective layer is normally removed by etching in the region free of the ink on the substrate surface.
When a resin is used for forming the discharge port, it is little adhesive to the second protective layer of Ta or the like, causing the discharge port to easily come off the second protective layer. One of the proposals to solve the above problem is use of an adhesive layer of polyether amide or the like between the substrate coated with the second protective layer of Ta or the like and material that forms the discharge port, in order to improve adhesion between them, as disclosed by Japanese Patent Application Laid-Open No. 11-348290.
The ink jet head generally comprises an electric wiring on the heat generating resist layer, as described earlier, and one or more steps tend to be formed between the electric wiring and the heat generating resist layer. Thickness of the layer tends to be uneven around such a step, and the layer must be formed in such a way to sufficiently cover the step and prevent exposure of the wiring or resist layer it protects. When coverage of the step (hereinafter referred to as xe2x80x9cstep coveragexe2x80x9d) is insufficient, the exposed portion of the heat generating resist layer may directly come into contact with the ink, to possibly cause problems, e.g., electrolysis of the ink, and reactions between the ink and a material which constitutes the heat generating resist layer to eventually destroy the resist layer. Such a step tends to cause uneven layer thickness, which, in turn, may cause partial concentration of thermal stresses produced in the protective layer as it is repeatedly exposed to heat, and eventually cracking of the protective layer. These cracks or pinholes, if formed, may allow inflow of the ink, to eventually destroy the resist layer.
Conventionally, attempts have been made to solve these problems by increasing thickness of the protective layer to improve step coverage and reduce pinholes. Increasing the thickness does contribute to step coverage and reduced pinholes, but causes new problems, described below, resulting from retarded supply of heat to the ink.
The heat generated in the heat generating resist layer is transferred to the ink via the protective layer. Increased thickness of the protective layer increases thermal resist of the working plane of heat, i.e., the space between the protective layer surface and heat generating resist layer. As a result, the resist layer needs an excessive power load, which is disadvantageous from power saving. In addition, accumulation of excessive heat in the substrate deteriorates its thermal response, and consumption of excessive power deteriorates durability of the heat generating resist layer.
These problems are mitigated as thickness of the protective layer decreases. However, when the protective layer is formed by sputtering or evaporation, which is a normal film-making procedure for producing the conventional ink jet head, decreasing the thickness is substantially limited by the problems associated with insufficient step coverage which deteriorates durability of the resist layer, as described above.
It is known that an ink used for printing by an ink jet head as described above will have improved foaming stability as it is heated at a higher rate. More concretely, electrical signals to be applied to thermal energy generating means are normally rectangular delayed pulses, and decreasing the pulse width improves the foaming stability, thereby improving discharge stability of the flying ink droplets and hence print quality. However, the conventional ink jet must have the protective layer of certain thickness as described above, and excessive heat must be generated by the thermal energy generating means to overcome increased thermal resist of the protective layer, resulting in deteriorated durability and thermal response. This naturally imposes limitations on decreasing pulse width and improving print quality.
Referring to FIG. 17, which shows the sectional structure of the conventional ink jet head on the substrate side, the oxide layer 12 formed on the surface portion of the substrate 11 is coated with the heat generating resist layer 13 by sputtering, on which at least one pair of the electrode wiring circuits 14a and 14b are formed, where the step 15, standing on the heat generating resist layer 13, is formed by the presence of the circuit 14a or 14b. 
In such a configuration, defects, e.g., pinholes, tend to be formed in the lower, first protective layer 17 coated with the second protective layer 16, in particular around the step 15, where the circuit 14a or 14b tends to be exposed. In order to avoid the above problem, thickness of the first protective layer 17 is increased to an excessive extent (normally at least twice as much as that of the circuit 14a or 14b) to secure sufficient coverage for the step 15.
Some proposals to decrease thickness of the first protective layer 17 without deteriorating step coverage include use of bias sputtering, known for its good step coverage, as the film-making procedure to form the first protective layer 17, as disclosed by Japanese Patent Application Laid-Open No. 60-234850. Japanese Patent Application Laid-Open Nos. 62-45283 and 62-45237 propose the methods to improve step coverage by changing step shapes after the first protective layer 17 is formed using etchback or sputter etch. Japanese Patent Application Laid-Open No. 62-45286 proposes reflow of the protective layer to improve step coverage.
However, bias sputtering involves disadvantages of insufficient stability of film thickness it gives and generation of dust around the target. Etchback, sputter etch and reflow need increased number of processes and push up cost.
Another method, disclosed in HP Journal, May, 1985, proposes tapered sectional structure of the electrode wiring circuits 14a and 14b, as shown in FIG. 18, thereby improving step coverage of the protection layer 15 showing a section of a substrate structure for a conventional ink jet head. Still another method proposed uses a developer as an alkaline solution for simultaneously etching the circuits 14a and 14b and resist. The members shown in FIGS. 17 and 18 are marked with the same numbers, when they are functionally corresponding to each other.
However, the tapered section of the step 15, produced by these methods, is insufficient in uniformity and reproducibility, tending to change area by area, and these methods are particularly disadvantageous for the large-size substrate 11. In particular, insufficient uniformity of the tapered section of the step 15 will cause the following problems:
Step coverage tends to be insufficient at a sharp angle of the tapered section of the step 15, causing the above-described problems. At a gentle angle of the tapered section, on the other hand, the circuits 14a and 14b have a smaller width and cross sectional area than the other parts at a sharper angle, to have a higher electrical resistance. As a result, the circuit for the ink jet head has a fluctuation of a resistance distribution, and, when used for a printer, may deteriorate print quality or the like.
Still another method proposes a different sectional structure of the conventional ink jet head around the substrate, shown in FIG. 19, which has been already commercialized. The protective layer 15 on the heat generating resist layer 13 is selectively thinned. The members shown in FIGS. 17 and 19 are marked with the same numbers, when they are functionally corresponding to each other.
However, this method needs an increased number of steps, because 2 steps are needed for forming the protective films 15a 15b as the insulator after the electrode wiring circuits 14a and 14b are formed, and the region to be selectively thinned is exposed to light using a photomask resulting in the increased steps. Furthermore, selectively thinning the inside of the heat generating region decreases thermal efficiency at its outer periphery.
It is an object of the present invention to provide a substrate unit for liquid discharging head with improved stability to foaming, which results from rapid heating of the ink, and durability by decreasing, as far as possible, thickness of the first protective layer on the heat generating resist layer, decreasing thickness of the electrode wiring circuit and its layer for the heat generating resist layer, and removing the second protective layer without damaging the first protective and wiring layers.
It is another object of the present invention to provide a highly reliable liquid discharging head, which incorporates the above substrate unit, with improved adhesion between the substrate with exposed protective layer of Ta or the like and member which forms the discharge port.
It is still another object of the present invention to provide a method for forming the above substrate unit for liquid discharging head, and also to provide a cartridge and image forming apparatus.
The substrate unit for liquid discharging head of the present invention is for a head which gives thermal energy to the liquid for film boiling, to discharge droplets of the liquid from its discharge port. It comprises an electrothermal transducer provided on the substrate surface to generate thermal energy, a pair of electrode wiring circuits provided on the substrate surface and connected to the transducer, first protective layer formed over the substantially entire surface of the substrate to cover a pair of the electrode wiring circuits and transducer, and second protective layer formed over the first protective layer to cover the transducer and the area in which the transducer is connected to the wiring circuit, wherein a pair of the electrode wiring circuits are 1800 to 2400 xc3x85 thick, and the portion of the first protective layer covered by the second protective layer is 2600 to 3400 xc3x85 thick and thicker than the portion of the first protective layer not covered by the second protective layer.
The present invention has the first protective layer which is made uniformly thin while securing sufficient step coverage on the area where a step is formed by the wiring circuit to stand on the protective layer, without needing an additional film-making step or use of mask.
A pair of the electrode wiring circuits for the substrate unit for liquid discharging head of the present invention are preferably 2000 to 2200 xc3x85 thick, and the portion of the first protective layer covered by the second protective layer is preferably thicker than the one not covered by the second protective layer by 100 to 200 xc3x85. The first protective layer is preferably at least 1.08 times thicker than the wiring circuit.
The method of the present invention is for producing the substrate unit for liquid discharging head which gives thermal energy to the liquid for film boiling, to discharge droplets of the liquid from its discharge port. It comprises (a) a step of forming an electrothermal transducer on the substrate surface to generate thermal energy, (b) a step of forming a pair of electrode wiring circuits on the substrate surface in such a way to connect them to the transducer, (c) a step of forming a 2600 to 3400 xc3x85 thick, first protective layer over the essentially entire surface of the substrate to cover a pair of the electrode wiring circuits and transducer, and (d) a step of forming, by dry etching, a second protective layer over the first protective layer to cover the transducer and the area in which the transducer is connected to the wiring circuit, in such a way to keep the portion of the first protective layer not covered by the second protective layer thinner than the one covered by the second protective layer, and also to keep thickness of the electrode wiring circuits at 1800 to 2400 xc3x85.
The method of the present invention prevents, while forming the second protective layer by dry etching, damages of the transducer and a pair of the wiring circuits.
In the method of the present invention for producing the substrate unit for liquid discharging head, the second protective layer may be formed by dry etching in such a way to keep thickness of the etched portion of the first protective layer not covered by the second protective layer at 100 to 200 xc3x85.
The liquid discharging head of the present invention comprises a discharge port from which the liquid is discharged, electrothermal transducer on the substrate surface to generate thermal energy for causing film boiling of the liquid, a pair of electrode wiring circuits provided on the substrate surface and connected to the transducer, first protective layer formed over the essentially entire surface of the substrate to cover a pair of the electrode wiring circuits and transducer, and second protective layer formed over the first protective layer to cover the transducer and the area in which the transducer is connected to the wiring circuit, wherein a pair of the electrode wiring circuits are 1800 to 2400 xc3x85 thick, and the portion of the first protective layer covered by the second protective layer is 2600 to 3400 xc3x85 thick and thicker than the portion of the first protective layer not covered by the second protective layer.
In one embodiment of the substrate unit of the present invention for liquid discharging head, the discharge ports are formed at intervals of 600 dpi in at least two rows running in parallel to each other, wherein each port in one row may be off the corresponding one in the other row by half a pitch. The liquid may be an ink and/or a treatment solution for adjusting printability of the ink on a printing medium. The liquid may be discharged by driving pulses given to the electrothermal transducer from the discharge port at 5 picoliter or less.
The substrate unit of the present invention for liquid discharging head may have an additional member for forming the discharge port, which is joined with portion of the second protective layer and portion of the first protective layer not covered by the second protective layer via an adhesive layer.
In the above design, the above portions of the first and second protective layers are preferably tapered in the vicinity of the boundary between the portion of the first protective layer covered by the second protective layer and the one not covered by the second protective layer. It is also preferable that the first protective layer is tapered at a sharper angle than the second protective layer. The adhesive layer may be of polyether amide resin, which may be thermoplastic. The member for forming the discharge port may be of resin, which may be hardened epoxy resin by cationic polymerization.
In the substrate unit of the present invention for liquid discharging head, the discharge port may be formed to face the electrothermal transducer.
The cartridge of the present invention comprises a liquid discharging head and liquid tank storing the liquid to be supplied to the liquid discharging head, wherein the liquid discharging head comprises a discharge port from which the liquid is discharged, electrothermal transducer on the substrate surface to generate thermal energy for causing film boiling of the liquid, a pair of electrode wiring circuits provided on the substrate surface and connected to the transducer, first protective layer formed over the essentially entire surface of the substrate to cover a pair of the electrode wiring circuits and transducer, and second protective layer formed over the first protective layer to cover the transducer and the area in which the transducer is connected to the wiring circuit, a pair of the electrode wiring circuits being 1800 to 2400 xc3x85 thick, and the portion of the first protective layer covered by the second protective layer being 2600 to 3400 xc3x85 thick and thicker than the portion of the first protective layer not covered by the second protective layer.
The cartridge of the present invention may incorporate the liquid tank in such a way that it can be releasably attached to the liquid discharging head.
The cartridge of the present invention may have an additional member for forming the discharge port, which is joined with portion of the second protective layer and portion of the first protective layer not covered by the second protective layer via an adhesive layer.
The image forming apparatus of the present invention is provided with a section of attaching a liquid discharging head comprising discharge ports from which the liquid is discharged, an electrothermal transducer on the substrate surface to generate thermal energy for causing film boiling of the liquid, a pair of electrode wiring circuits provided on the substrate surface and connected to the transducer, first protective layer formed over the essentially entire surface of the substrate to cover a pair of the electrode wiring circuits and transducer, and second protective layer formed over the first protective layer to cover the transducer and the area in which the transducer is connected to the wiring circuit, wherein a pair of the electrode wiring circuits is 1800 to 2400 xc3x85 thick, and the portion of the first protective layer covered by the second protective layer is 2600 to 3400 xc3x85 thick and thicker than the portion of the first protective layer not covered by the second protective layer.
In the image forming apparatus of the present invention, the section of attaching the liquid discharging head may have a carriage which can be scanned in the direction intersecting the direction of travel of the printing medium onto which the liquid is discharged from the discharge port. The liquid discharging head may be releasably attached to the carriage by attaching/detaching means.
The cartridge of the present invention may have an additional member for forming the discharge port, which is joined with portion of the second protective layer and portion of the first protective layer not covered by the second protective layer via adhesive layers.
The present invention can give the uniformly thin first protective layer, without needing an additional film-making step or use of mask, by providing the first protective layer formed over the substantially entire surface of the substrate to cover a pair of the electrode wiring circuits and transducer, and second protective layer formed over the first protective layer to cover the transducer and the area in which the transducer is connected to the wiring circuit, while keeping a pair of the electrode wiring circuits 1800 to 2400 xc3x85 thick, and the portion of the first protective layer covered by the second protective layer 2600 to 3400 xc3x85 thick and thicker than the portion of the first protective layer not covered by the second protective layer. It also secures sufficient step coverage on the area where a step is formed by the wiring circuit to stand on the substrate.
The present invention therefore allows the liquid to be heated rapidly to improve foaming stability, leading to saved power for the liquid discharging head and controlled heating, which, in turn, prevents accumulation of heat in the substrate, improves its thermal response, and realizes printing works of high quality at higher driving frequency.
In particular, the first protective layer can be uniformly thinner when thickness of a pair of electrode wiring circuits is kept in a range from 2000 to 2200 xc3x85. Such a thin protective layer will have improved durability, because partial concentration of thermal stresses, caused by cyclic generation of heat, is prevented; otherwise, it may be cracked to allow inflow of the liquid which can disconnect the wiring circuit and eventually destroy the heat generating resist layer. The electrothermal transducer and electrode wiring circuit can be kept intact, when the first protective layer is coated by dry etching with the second protective layer to cover the transducer and the area in which the transducer is connected to the wiring circuit, in such a way to keep the portion of the first protective layer not covered by the second protective layer thinner than the one covered by the second protective layer, and also to keep a pair of the electrode wiring circuits 1800 to 2400 xc3x85 thick.
Tapering those sections of the first and second protective layers covered by the member for forming the discharge port improves adhesion between the first protective layer and member for forming the discharge port in the vicinity of the second protective layer, efficiently preventing exfoliation of the member.
Therefore the first protective layer exposed to the electrothermal transducer, a pair of the electrode wiring circuits and driving device provided on the substrate, as a result of removal of the second layer, can sufficiently exhibit its functions in the presence of moisture or in a high humidity atmosphere.
A highly functional liquid discharging head of 1200 dpi can be obtained by forming the discharge ports at intervals of 600 dpi in at least two rows running in parallel to each other, in such a way that each port in one row is off the corresponding one in the other row by half a pitch.
A high-quality image of high resolution can be produced, when driving pulses given to the individual electrothermal transducer is adjusted to discharge the liquid at 4 picoliter from the discharge port.